A conventional TTL-SIR type AF sensor has been disclosed in detail in “CMOS Linear Auto-focus Sensor for 7-point Wide Area Auto-focus System”, ITE Technical Report, Vol. 25, No. 28, pp. 1-6, 2001, or the like by the applicant of the present invention.
FIG. 9 shows a layout of linear sensors of such a solid-state image pickup device for AF. To perform the detection of 7 points including the center cross detection, eight linear sensor pairs 131 are arranged on a same semiconductor substrate 130 in correspondence to the detecting points. By arranging two kinds of linear sensor pairs in the horizontal and vertical directions in a crucial shape with respect to the center detecting point, the cross AF for executing both of the vertical line detection and the lateral line detection can be realized. In those devices, a light beam which passed through a photographic lens is guided to a linear sensor 132 for a base portion and a linear sensor 133 for a reference portion arranged on the AF sensor. Two object images are formed again at two positions on the linear sensors 132 and 133 by a secondary image forming optical system and a phase difference between those two object images is detected, thereby obtaining a defocusing amount (actually, by executing a correlation arithmetic operation of a signal of the base portion and a signal of the reference portion, resolution of focusing precision is improved). Detecting precision in the above system largely depends on a pixel pitch and a base-line length of the linear sensor (distance between an optical center of the linear sensor for the base portion and that of the linear sensor for the reference portion). Generally, if the pixel pitch is small and the base-line length is large, the focusing precision can be raised more. However, if the base-line length is increased, a chip size and a size of optical unit also increase. Therefore, if the pixel pitch is decreases, it is more effective for miniaturization of the camera.
Several techniques of improving the focusing precision by another method have been also disclosed. A focus detecting apparatus of a camera which has a sensor array of a large pixel pitch and a sensor array of a small pixel pitch and in which, when a focus detection is disabled in one sensor array, the focus detection is executed by using an output of the other sensor array is disclosed in Japanese Patent Application Laid-Open No. S64-80920. An apparatus in which, as a sampling pitch for sampling an object image, a plurality of kinds of sampling pitches including an initial state can be selected and, by switching the pixel pitch in accordance with a spatial frequency of an object and executing the focus detection, the focusing precision can be improved has also been disclosed in Japanese Patent Application Laid-Open No. H11-14900. The focusing precision can be improved by providing one more linear sensor other than the linear sensors which are ordinarily used or by changing the pixel pitch. A capturing ratio (ratio at which the object can be focused) of the object is also improved.
In the above conventional techniques, however, since there is sensitivity distribution also in the pixel, there is a case where the precision deteriorates in dependence on the image forming position of the object image. The case where the focusing precision deteriorates will be described hereinbelow.
FIG. 10 is a diagram showing sensitivity distribution within a cell along the horizontal direction in the pixel of the general AF sensor. The sensitivity at the center of each of photodiodes 110-1 to 110-5 serving as pixels is the highest. As the position approaches the pixel edge, the sensitivity deteriorates and the sensitivity in an isolation region 128 is low. FIG. 11 shows a relation between an object image formed on the photodiode 110-3 and outputs from the AF sensor regarding the photodiodes 110-1 to 110-5. In FIG. 11, the object image is formed at almost the center of the photodiode 110-3, so that the output of the photodiode 110-3 is the largest. The signals of the photodiodes 110-2 and 110-4 adjacent to the photodiode 110-3 are outputted at a certain ratio due to crosstalks from the photodiode 110-3. If the object image is formed in the isolation region between the photodiodes 110-3 and 110-4 as shown in FIG. 12, since magnitudes of the outputs of the photodiodes 110-3 and 110-4 are almost equal, it is determined to be unspecified in which pixel on the right or left side of the isolation region a peak of the object image is located. Thus, even if the same object is auto-focused, an arithmetic operation result differs every time. To reduce such an influence, it is preferable to reduce the horizontal pixel pitch. For example, if the pixel pitch is reduced to half, a focusing error is also reduced to about half. However, since the sensitivity deteriorates if the pixel pitch is simply reduced, there is a case where the AF itself cannot be performed at the time of low luminance. To reduce the pixel pitch without deteriorating the sensitivity, it is required to introduce a fine-patterning process. However, since a long developing period of time and very high developing costs are necessary to accomplish the fine-patterning process, it is difficult to develop the solid-state image pickup device for the AF with low costs in a short period of time.